Can I run Qwen3-8B on an RTX 4090?

Yes

Its published weights (16.4 GB) fit in the 21.6 GB an RTX 4090 can use, with room for ≈ 35K tokens of context.

hf download Qwen/Qwen3-8B

Files and context on RTX 4090

Each file of Qwen3-8B (and of GGUF versions the site has analyzed), and how much context fits next to it, with the context cache in 16-bit or 8-bit.

Qwen3-8B on other hardware

Popular models that run on RTX 4090

Assumes 90% of the card's memory is usable and at least 4K tokens of context for one conversation, with the context cache in 16-bit unless noted. Fits leaves at least 20% of that memory for context; Tight leaves less.